Method for making sintered silicon nitride articles

ABSTRACT

In the manufacture of sintered silicon nitride articles by hot isostatic pressing, multiple silicon nitride parts are encapsulated within a single capsule for the pressing operation. Suitable spacers placed between the parts ease separation thereof after sintering.

This invention is concerned with the manufacture of sintered siliconnitride articles by means of hot isostatic pressing, such as is shown inU.S. Pat. Nos. 4,351,858, 4,256,688 and 4,112,143 and British Pat. No.1,522,705. The patents disclose (1) the pressing of silicon nitridepowder into a preform, (2) encapsulating the preform in a material thatis impermeable to the pressure medium (usually a gas) that is used inthe hot isostatic pressing operation and which is plastic at the hotisostatic pressing conditions, and (3) hot isostatic pressing theencapsulated preform. The patents disclose hot isostatic pressing ofonly a single part at a time. This invention is concerned with the hotisostatic pressing of more than one part at a time, for reasons ofeconomy of production.

In this invention, two or more preforms are stacked together and arethen encapsulated within a single capsule to be hot isostaticallypressed. In order to prevent the preforms from diffusion-bonding to eachother during hot isostatic pressing, suitable refractory spacers areplaced therebetween. The spacers must be inert with respect to thesilicon nitride preforms and must have a melting point greater than thehot isostatic pressing temperature. If the spacer material, for example,alumina, is insufficiently inert with respect to the silicon nitridepreforms, the alumina spacer may be coated with boron nitride to providethe desired inertness. In some cases it may be desirable to completelywrap the stack of preforms in a suitable metal foil prior toencapsulating the stack in a capsule, the reason being to prevent anyloose particles of powder that may be present on the stack frominterfering with the formation of a sealed capsule.

In the drawing,

FIG. 1 shows a stack of preforms separated by spacers.

FIG. 2 shows the stack wrapped in metal foil.

FIG. 3 shows several spaced-apart wrapped stacks within an open-endedglass tube.

FIG. 4 shows the glass completely collapsed around each stack.

FIG. 5 shows an encapsulated stack cut apart from the others and readyfor hot isostatic pressing.

In one example of this invention, cutting tool blanks, or preforms 1,were pressed from silicon nitride powder having an average particle sizeof about one micron and containing about 6% of an organic binder, forexample, stearic acid and/or carbowax, in a die at a pressure of about50,000 psi. Preforms 1 were 1/2 inch square by 3/16 inch thick. Thepurpose of the binder was to give the preforms serviceable greenstrength. Spacers 2 were then pressed in the same die at about the samepressure from boron nitride powder having an average particle size ofabout one micron and containing about 10% of the same binder. Spacers 2were 1/2 inch square by 50 mils thick. Preforms 1 and spacers 2 werethen heated at 550° C. for 2 hours to drive off the binders. Stack 3 wasthen formed by assembling six preforms 1 with spacers 2 separating thepreforms from each other, as shown in FIG. 1. In order to give stack 3adequate strength for handling, a boron nitride coating was appliedbetween each preform 1 and spacer 2 and was applied over the outside ofstack 3. The boron nitride coating was applied from a suspension of fineboron nitride particles in an aqueous binder solution. Stack 3 was thenheated at 550° C. for 72 hours to drive off the water and binders and tothoroughly dry the stack. The height of stack 3 was about 11/2 inches.Stack 3 was then wrapped in a 21/2 inch by 5 inch sheet of 1/2 milmolybdenum foil 4 so that all six surfaces of stack 3 were covered.Stack 3 was then inserted into horizontal glass tube 5 which was sealedat one end. The inside diameter of glass tube 5 was about 2 mm greaterthan the largest cross sectional dimension of stack 3; the length ofglass tube 5 was 36 inches. Glass tube 5 was made of a glass of highsilica content such as Vycor. Ten foil wrapped stacks 3 were insertedinto glass tube 5 leaving a space of about 11/2 inches between stacks 3.Glass tube 5 was then carefully inserted into a furnace in a horizontalposition, in order to maintain the 11/2 inch spacing between stacks 3,and was heated to the softening point of the glass, about 1400° C.,while a vacuum of about 500 microns was drawn on the open end of glasstube 5. After the glass collapsed around each stack 3, as shown in FIG.4, the collapsed tube was removed from the furnace and cooled. Aftercooling, the collapsed tube was cut through at spaces 6 between stacks 3with a diamond saw, leaving each stack 3 enclosed in a sealed glasscapsule 7, as shown in FIG. 5. Each capsule 7 was then hot isostaticlypressed, as shown in the above mentioned patents, in an argon atmosphereat a maximum temperature of 1800° to 1900° C. and a maximum pressure ofabout 20,000 psi. After cooling, capsule 7 was removed by breaking it,metal foil 4 was removed from around stack 3, and cutting tool blanks 1were easily separated from each other because of spacers 2 inbetween.

The silicon nitride may contain sintering-promoting additives such asmagnesia as disclosed in the above mentioned patents, or other propertymodifiers, for example, titanium carbide to modify hardness or aluminato modify thermal conductivity.

Other refractory materials that may be used for spacer 2 are alumina,spinel, magnesia, zirconia and rare earth oxides, for example, lanthana,yttria and ceria. In the event that such materials are insufficientlyinert with respect to silicon nitride, the boron nitride coating theronwill prevent direct contact with the silicon nitride preform and therebyprevent direct contact with the silicon nitride preform and therebyprevent reaction therebetween.

If desired, only one stack 3 need be encapsulated at a time. In such acase, glass tube 5 could be shorter than when multiple stacks areencapsulated in one glass tube.

We claim:
 1. In the process of making a sintered silicon nitride articleby pressing a preform from silicon nitride powder and then isostaticallypressing the preform at high temperature and high pressure to make thesintered silicon nitride article, the improvement which comprises thesteps of assembling a plurality of such preforms in a stack with spacersseparating the preforms from each other, the spacers comprising materialthat is inert with respect to the silicon nitride preforms and having amelting point above the isostatic pressing temperature, enclosing thestack in a sealed glass capsule, hot isostatically pressing the glasscapsule, removing the glass from around the stack, and then separatingthe sintered silicon nitride articles from each other, the separatingstep being eased because of the spacers.
 2. The process of claim 1wherein the spacers have the same shape as the preforms but are thinner.3. The process of claim 1 wherein the preforms are made by pressing thesilicon nitride powder in a die and the spacers are made by pressing thespacer material in powder form in the same die.
 4. The process of claim1 wherein the spacer boron nitride.
 5. The process of claim 1 whereinthe spacer material comprises spinel coated with boron nitride.
 6. Theprocess of claim 1 wherein the spacer material comprises alumina coatedwith boron nitride.
 7. The process of claim 1 wherein the spacermaterial comprises magnesia coated with boron nitride.
 8. The process ofclaim 1 wherein the spacer material comprises zirconia coated with boronnitride.
 9. The process of claim 1 wherein the spacer material comprisesa rare earth oxide coated with zirconia.